Unforgivable
by OutCold
Summary: It was sex. It was love. It was wrong. It was right. It was tragic. It was brilliant. And somewhere, beneath it all, they both knew: It was unforgivable. The story of Mark and Addison's affair from the beginning until she leaves for Seattle. Maddison.
1. What We're Feeling

**Disclaimer: Well, I created Adrienne. But y'know, it's pretty obviously not mine.**

**A/N: First Grey's fic! Maddison. I'm a die hard Mer/Der, but I'm flexible on Mark and Addison - they have great relationships with other people too. I do love the ship though, and the characters are just priceless. Their story interests me, and so, here it is the way I imagine it.**

**

* * *

**

"Honey, it's perfectly normal to look at other guys when you're a married woman," Adrienne reassured.

"Yeah, I suppose," Addison said, slumping down.

Adrienne was the best friend she had, but Addison couldn't even tell her the full story. It was just too awful to pass on, and Addie knew that if you want to keep a secret, you can't tell anyone. Not a single soul. Not that that stopped her usually – usually she'd tell Derek. She could tell Derek everything. Except this. She especially couldn't tell Derek, of all people, this.

She knew that it was normal to look at other guys. I mean, no woman was ever going to stop looking at men. Not even Derek, who she knew had a bit of a hidden jealous streak, would ask that. And if it were just a man, it wouldn't be a problem. If it were any man except him.

Mark.

And even if it were only looking. Looking was acceptable – damn near every woman in the hospital _looked _at Mark. But she'd catch herself making the occasional inappropriately flirtatious comment, brushing her hand against his arm. And every time she did it, he'd smile in that infuriating way, like he knew. Well of course he knew. Much as she would have liked to think otherwise, it wasn't exactly subtle.

"Addie? Penny for them?" Adrienne said, interrupting.

"I was just thinking my break ends in five minutes, I should go."

The two women were sitting cross-legged on the lower bunk of an on-call room bed, Addison on a quick break and Adrienne, a psychiatrist, skiving because she didn't have any appointments. Addison swung her legs off and grabbed her white coat from where it was hanging up.

"Thanks for the free therapy," she grinned.

"Anytime, honey," Adrienne said, kicking back so she was lying on the bed. "I'm going to have a nap."

Addison laughed and left the room. Maybe it was because she was still laughing that she didn't see him, but for whatever reason she walked straight into Mark.

"Woah," he said, grabbing her shoulders to steady them both and letting his clipboard drop to the floor. "Easy there."

To her horror, Addison felt her cheeks blush a deep red, and she stammered a little over her apology. Once she'd managed to get out 'sorry, wasn't looking', she shut her mouth and bent down to pick up his chart for him, at the same time as he did. They fumbled together for a second or two, fingers brushing, embarrassment climbing, until eventually they lifted the board and stood.

"Well," Mark said. "I should go see my patient."

"Yeah," Addison replied. "Me too.""

She realised she was still holding the other end of the clipboard and let go as though it were burning hot, taking a step back. They looked at each other, simultaneously opening their mouths to speak.

"Anyway," they said in sync, turning and walking in opposite directions as quickly as they could without drawing suspicion.

Once Mark was far enough away he did the first thing he thought of – hid in a supply cupboard. Pushing out some interns who'd luckily just found what they needed, he slammed the door shut and rested his back against it, breathing heavily as though he'd been running. He couldn't help it. He'd tried, he'd tried so much. She was his best friend's wife, for god's sake. She was Derek's. Her bright smile, her red hair, the blush that had rendered him speechless minutes before, the touch of her hand – they were Derek's. But recently Mark had been looking at her differently. Recently his heart had skipped when she'd seen her approaching, his palms had sweated when he was forced to spend time with them as a couple, watch them holding hands, smiling together, like everything was perfect. And for Derek, surely it was. They'd always competed, and were both tops of their respective fields in surgery. They'd had 'who can get the most girls' competitions when they were younger. Once it would have given Mark pleasure to say he'd usually won. But now Derek had the prize – Derek had Addison, and no amount of strangers, nurses or girls from bars could equal that.

He'd never dream of making a move, of course. He could never hurt Derek like that; they were practically brothers. It would be so much easier, though, if Addison would stop. Stop making suggestive comments, stop putting her hand on his arm or smiling at him for no reason at all. At least, that was what it seemed like. At the least, it was all in his head, wishful thinking. At the most, it was the lightest of playful flirting. She'd never do it if she knew what he thought... felt... imagined. If she knew... he shuddered at the thought.

Derek and Addison didn't have a perfect marriage, Mark knew better than anyone. But they were close, still, and if she knew perhaps he would too. And that, that would be unbearable.

Mark was used to breaking taboos. He'd never considered himself bound by the rules of society. But he knew in the depth of his heart that the things he was thinking were unforgivable. And it wasn't because of any social nicety – he was dreaming, fantasising, of betraying someone he loved more than his own life.

He'd tried to exorcise himself of the traitorous feelings, but nothing worked. It didn't matter what he did, every day – and every night – she haunted him. When he felt ready, he stepped out of the cupboard. He had one more patient to see and that should last him until the end of his shift. Chuckling to himself, he wondered what the Chief of Surgery would think if he knew that he paid Mark to daydream about a married woman and hide in cupboards.

He decided that after work he'd go out to try and pick up a girl for the night. If he failed then, the alternative was always to get blindingly drunk.

He knew from experience it wouldn't work, that the next day when he saw Addison again the feelings would be back, but for the night, just that night, he wanted to make them go away.

* * *

***goes to bite nail* *reminds herself she's given up biting her nails* *types what she's doing to stop herself biting her nail***

**What did you guys think?**


	2. What We're Dreaming

**Disclaimer: *yawn* I did literally yawn at that moment, but it wasn't really because of the disclaimer. :P**

**A/N: I personally like how I had this laid out on Word, with Mark's bit in this masculine square-ey capital-ey font, Addison's in Monotype Corsiva, and the end bit in Bradley Hand. But there you go, I'm very visual for a writer. **

**

* * *

**

Mark went to a bar straight from the hospital. It wasn't the one that he, Derek, Addison and Adrienne frequented, but he had been there alone a few times before. The barmaid served him his beer with a small smile, and left him to drink it in peace. Something in him craved a stronger drink, but a whiskey or scotch tended to put girls off. If he were unsuccessful, he'd have one when he got home. But he didn't expect to be unsuccessful.

In fact, it only took half an hour of flirting with the girl who'd served him (Natalie) for Mark to think they were at the stage he could ask when she got off.

"Hour and a half," she shrugged, obviously expecting this to put him off.

"I can wait," Mark replied with a charming grin.

This provoked a genuine smile from Natalie, which delighted him. They continued to chat whenever she had a spare second, and he felt pretty good about himself by the time they walked out the door together, his arm wrapped around her waist. He successfully ignored that this wasn't what he really wanted. Well, sort of successfully. In a way. He took her back to his apartment and asked if she wanted to drink. Natalie smirked and shook her head.

"That seems like a waste of time and alcohol," she said, tugging Mark forward by his belt loops. She smelt faintly of perfume and the bar that she worked in. Naturally, Mark responded, moving through the stages of the ritual that he knew so well. He'd never been more relieved not to be alone, even if it wasn't Natalie he really wanted to be with. As he wound her dark hair around his body, it flashed red in his eyes, and when he closed them he couldn't remember the face of the woman he'd taken home – but there was one face he could picture more clearly than he'd ever admit.

When they were lying together afterward, Mark couldn't look at her, and there was only one reason, one awful reason... He wanted to pretend it was Addison lying beside him. It was wrong that he'd done it because of Addison, wrong that he'd imagined it was Addison, _so _wrong that he wanted it to be, even now, after the act, Addison who was sharing his bed. Mark Sloan wallowed in guilt and self-loathing. He didn't sleep.

Derek was working a late shift, but Addison didn't want to go to bed without him. It had been like this for months, working mismatching hours, only really seeing each other in scrubs and white coat. It had begun to bother her a little, but she'd never thought to do anything about it – until today. Her moment with Mark in the corridor had left her with two things: a thrilling feeling of electricity running through her body, and a deep unsettling guilt. Somehow she thought the act of going to bed at the same time as her _husband _might finally remind her, really remind her, that she was married. So she curled up on the sofa, waiting for Derek to get home.

It almost surprised her when he was on time, at one in the morning. She knew the hospital, the unexpected surgeries, the emergencies, and if he wasn't home by two, she'd promised herself to go to bed, but she was relieved he was back. For some reason, the door clicking shut made her nervous. Then she realised – she felt adulterous. Which was just stupid. She hadn't done anything. And armed weakly with that knowledge, she went to meet Derek. He looked surprised to see her awake.

"Shouldn't you be in bed?"

"Couldn't sleep," she lied, "so I thought I'd wait for you. Good shift?"

"I've had worse," Derek smiled. "We'll have to wait until tomorrow to know for sure how it all went, but yeah. It was good."

She smiled back and noticed his expression change, become gentler, more concerned. He put his arm around her and let her rest her head on his shoulder.

"So what's up, Addie? Why can't you sleep?"

She made a non-committal mumbling sound, and he stroked her arm gently. On impulse, Addison kissed him.

She knew her husband was perfect. Perfect looks, perfect surgeon, perfect lover. He knows her body, she knows his, and together they are a well-oiled, perfect machine. But tonight... tonight it isn't good enough for her. Tonight, despite the fact that he kisses her in places that make her weak at the knees, whispers to her all the things she loves to hear, it's missing something. She has a fleeting thought that it's something to do with Mark, and knows instantly that she's right. Maybe it's other things too, but whatever happened with Mark today definitely contributes. She remembers the feeling of his arms holding her steady when she walked into him, of their fingers brushing over that damned clipboard, the charged air between them before they ran, yes, ran away. It feels like a betrayal to think of these things with Derek kissing her, Derek above her, but she can't stop, so she does the next best thing, and closes her eyes. When she's thinking of Mark, it's like her sensitivity is heightened, like all the nerves in her body are alight, but she's thinking of Mark and being with Derek, and she can't quite ignore how wrong that is. It works, but when it's all done with, she's left with a sick feeling in her stomach, guilt worse than the guilt she had before. Waiting up for Derek didn't solve a thing.

She curls into a ball and waits for him to fall asleep, which doesn't take long. He's exhausted, and so is she, but she gets out of bed and stumbles to the bathroom, bending over the toilet and heaving. She doesn't throw up, but only just, and tears are streaming down her cheeks. She knows she looks a mess, and also knows there are only four hours before she needs to get up, but even when she eventually gets to sleep, she doesn't sleep deeply or well.

The next morning, she covered up the dark circles as well as she could, and tries not to cry again. _I haven't done anything_, she told herself, but even as she thought it, she knew it wasn't true.

Addison saw Mark at the door of the hospital and sighed, trying to banish her thoughts from the night before and hoping her appearance was semi-decent. Mark saw her approach and smiled despite himself, though he was struck by the realisation that she didn't look well.

"You look tired, Addison," was naturally the first thing he said, in that teasing tone. "You and Derek have a late night?"

The thought of them together wasn't a particularly nice one for him, but they were married, so it wasn't exactly avoidable.

She thought of what she and Derek had done and blanched inwardly. For some reason she didn't want Mark knowing, didn't want to tell him.

"You really are a charmer," she said. "I didn't think I looked _that _bad, it was early actually. What about you? Pick up some barely legal little girl?"

In a way, the thought of Mark with someone else, confirming she was being stupid, was reassuring.

Mark remembered what he'd thought and felt with his 'barely legal little girl', who he was convinced had been at least twenty-five. For some reason he had a notion that he wouldn't actually be able to hide it, if he mentioned Natalie, she'd be able to read the whole story from his tone and expression. Besides, he really didn't want her to think that way of him, her joking comment stung a little.

"Ah, not last night," he smiled, standing to walk with her into the hospital.

* * *

**I know it's all build-up and stuff, I promise to get to, y'know, actual things happening soon. :)**


End file.
